The invention relates to a record carrier containing information in an optically readable information structure comprising trackwise arranged information areas which alternate in the track direction with intermediate areas, adjacent track portions differing from each other in that they comprise information areas of a first type and information areas of a second type respectively. The invention also relates to apparatus for reading such a record carrier.
Such a record carrier and apparatus are known, inter alia from Netherlands Patent Application No. 78 06378 which has been laid open to public inspection. The known record carrier may contain a television program, while the information being encoded in the frequency and/or the dimensions of the information areas in the track direction. These information areas are formed by pits pressed in the carrier surface. Except for those in the track direction, the dimensions of the information pits may be the same for the entire information structure. Alternatively, the information may be encoded in digital form, in which case the information areas also have the same dimensions in the track direction. A specific combination of the information areas and intermediate areas then represents a specific combination of digital zeros and ones.
In such optical record carriers, it is desirable to maximize the information density, and hence maximize the playing time, for example, of a recorded television program. For this purpose, the tracks would have to be arranged as closely as possible to each other. However, the distance between the tracks cannot be arbitrarily small. In known record carriers, in which the information areas of the adjacent tracks have the same geometry except for the dimension in the track direction, all the information areas influence the radiation of the read beam in the same manner. The read spot formed on the information structure by the read beam is a diffraction-limited radiation spot with a specific intensity distribution. The half-value diameter of this spot, i.e. the distance between two points in the spot where the intensity equals 1/e.sup.2 of intensity in the centre of the spot, is of the order of the track width. This means that, even in the case of a correct tracking some of the radiation of the read spot will be incident beyond the track being read, and may even be incident on adjacent tracks. The amount of radiation incident on the adjacent tracks increases as the track distance decreases. A portion of the radiation which is incident on the adjacent tracks and which is modulated by the information areas of these tracks can reach a radiation detector which receives the radiation modulated by the track being read. This effect, the crosstalk effect, determines the minimum distance between the tracks.
In Netherlands Patent Application No. 78 06378, which has been laid open to public inspection, it is proposed to increase the information density by making the information pits of the adjacent tracks, i.e. the information areas of a first and a second type respectively, of different depths and by reading these tracks with beams of different wavelengths. The depths and the wavelengths are selected so that the information pits of a first track produce maximum modulation in a beam of a first wavelength, while the information pits of adjacent, second, tracks have little influence on said beam, i.e.: are hardly observed by said beam. The pits of the second track, however, produce maximum modulation in a beam of a second wavelength, which beam in turn is hardly influenced by the information pits of the first track. The tracks can then be arranged substantially more closely to each other, without excessive crosstalk.
However, this arrangement presents some practical problems. Firstly, two radiation sources are required for producing two beams of different wavelengths, so that the read apparatus becomes more complex. Secondly, for adequately separated read-out of the two types of pits, comparatively deep pits, of the order of a few times the wavelength of their associated read beam, have to be formed with an accuracy of the order of a tenth of the wavelength of the read beam. This is a technologically difficult problem.